Electrolytic method of preparing ketoximes



United States Patent ELECTROLYTIC METHOD OF PREPARING KETOXIMES Christopher L. Wilson, Sloatsburg, N .Y. (207 Shadow Valley, High Point, N .C.)

No Drawing. Filed Oct. 14, 1954, Ser. No. 462,380

2 Claims. (Cl. 204-74) The present invention relates to an improved method of preparing aliphatic and cycloaliphatic ketoximes.

In the past, oximes have usually been prepared by first forming hydroxylamine by reduction of nitric or nitrous acids or their salts and then reacting the hydroxylamine with a carbonyl compound.

Ketoximes have also been prepared in a one-stage reduction process by mixing a ketone (acetone), a nitrite (sodium nitrite) and zinc dust, acetone oxime being the specific product formed. (A. Ogata and S. Hirano, J. Pharm. Soc. Japan, 50, 555, 1930.) The method has the disadvantage, however, of being fairly difficult to carry out commercially because of the use of metal dust.

One object of the present invention is to provide an improved process of preparing ketoximes.

Another object is to provide a process of preparing ketoximes that is well adapted for economical commercial use.

Still another object of the inventionis to provide a process of preparing ketoximes that produces a relatively high yield under economical production conditions.

A principal feature of the present invention is an improved process of preparing aliphatic and cycloaliphatic ketoximes by cathodically reducing an inorganic nitrite salt or nitrate salt in the presence of an aliphatic or cycloaliphatic ketone. The reduction is carried out under carefully controlled conditions of pH and temperature.

A specific example of preparation will now be given.

Example 1 19.6 g. of cyclohexanone is added to a solution of 25 g. of sodium nitrite in 250 m1. of water and this solution is electrolysed in the cathode compartment of a divided electrolytic cell using a zinc cathode having 100 sq. cm. of surface. While stirring the solution vigorously and admitting a continuous stream of carbon dioxide at a rate of 0.6 liter/min, a current of 2 amps. is used for a total of 20 ampere hours. During this process the temperature of the bath is maintained between and 12 C.

The solution is then brought to pH 5 by addition of hydrochloric acid and an ether extraction is performed. The ether extract contains some unused cyclohexanone, a small proportion of cyclohexanol and a major proportion of cyclohexanone oxime. The ether is evaporated and the residue remaining after the evaporation is distilled to drive off the cyclohexanone and cyclohexanol. The cyclohexanone has a boiling pt. or 46-53" C. at 15 mm. pressure. The cyclohexanone oxime remains behind in the distilling flask and is crystallised from pctroleum ether. A yield of 50.9% (11.5 g.) of the oxirne is obtained. It has a melting point of 8990.5 C. The current efliciency for oxime formation is 54.6%.

The principal reactions occurring in the above described process are the reduction of the nitrite to form hydroxylamine and the immediate condensation of hydroxylamine with cyclohexanone to form the oxime. Any soluble inorganic nitrite salt can be used in the process if the metal cation is not discharged under the con- 2 ditions of electrolysis. Examples of other suitable nitrites are potassium nitrite, calcium or barium nitrite, magnesium nitrite, etc. Any soluble inorganic nitrate salt can also be used, subject to the same limitations as for nitrites, suitable examples being those of the same metals mentioned immediately above. Besides cyclohexanone any other cycloaliphatic ketone can be used; for example cyclopentanone, methylcyclohexanone, menthone, cycloheptanone, cyclooctanone, etc. Any aliphatic ketone can also be used, for example, acetone, methyl ethyl ketone and, in fact, any other one even up to C Either aqueous or alcoholic solutions can be utilized in the cathodic reduction process. The alcohol must be miscible with water. Methyl, ethyl or isopropyl alcohols are useable. Other neutral solvents which are miscible with water may also be used; e.g., dimethylformamide, dioxane, ethylene glycol, etc.

The choice of the material of the cathode is of some importance in the process since some cathode materials will encourage undesirable competing reactions among which are: the further reduction of hydroxylamine to ammonia, the further reduction of oxime to amines, and the reduction of the ketone to alcohol. It has been found in the present invention that a zinc cathode gives the highest yields of oxime. Other suitable cathode materials, although less satisfactory than zinc, are magnesium, platinum, nickel, copper and lead. Cathodic reductions exactly similar to that described in Example 1 were carried out using, instead of the zinc cathode, cathodes of platinum, nickel, copper, lead and magnesium. The yield of cyclohexanone oxime was respectively 6.6%, 8.9%,11.1%, 24.1% and 13.3%.

As in most electrochemical processes, it is desirable to keep agitating the catholyte solution vigorously while the reduction process is continuing in order to obtain the highest product yield with most efiicient use of current. In the present process, it was found that vigorous stirring increased the yield of oxime by as much as 10%. Stirring can be conveniently accomplished by employing a cylindrical shaped cathode and rotating it rapidly. The zinc cathode of Example 1 measured 2.5 cm. in diameter and 6 cm. in length and a satisfactory rotation speed was 2000 rpm, although this was not critical.

The stream of carbon dioxide in Example 1 was employed to reduce the pH value to the proper level. The process of the present invention can be satisfactorily carried out between about pH 5 and 11. However, using a divided electrolytic cell, the progress of the electrolysis leads to the formation of alkali in the cathode compartment. The condensation reaction between the hydroxylamine, which is formed, and the ketone is favored by a pH of approximately neutral or slightly alkaline value. The introduction of the carbon dioxide at the proper rate neutralizes the alkali which is continuously formed and thus maintains the at the desired value. Other methods could, of coursebe used to maintain the pH Within the desired range, such as the use of bufier solutions (e.g., dipotassium hydrogen phosphate) or by adding sodium acetate, ethyl alcohol and an easily hydroizable ester such as ethyl acetate as mentioned by Law (J. Chem. Soc., 99,1113 (1911)).

Current density is not important except as it affects the temperature of the cell. Yield of oxime does not vary much as the current density is varied but, if too high currents are used, it becomes difficult to maintain a desirably low temperature. Thus, any convenient current density may be selected which does not cause an undesirable temperature rise in the solution. Current densities of between about 0.01 and 0.05 amperes/sq. cm., for example, have been found satisfactory.

Temperature control is important in order to get satisfactory yields of product. In practice it has been found that the temperature of the bath being electrolysed should be kept below about 70 C. and preferably below 20 C. The reason for maintaining the cathode bath at these temperatures is that too high temperature favors the undesired reduction of part of the ketone to alcohol, which thus reduces the yield of oxime.

For high yields the concentration of the nitrite or nitrate in the catholyte should be neither too high nor too low. It has been found that a convenient and satisfactory concentration of sodium nitrite is about by weight with reference to the solvent (water), although good results are obtained if this is doubled. High concentrations of dissolved salt has the disadvantage that it reduces the solubility of the ketone in the cathode medium. In general nitrite or nitrate concentration should be not higher than 20% and preferably about 10%. Such a concentration gives adequate conductivity. If the concentration of salt is too high, solubility of ketone is low, and if the salt concentration is too low, then conductivity is low and heating is pronounced.

Furter specific examples of preparing oximes in accordance with the present invention follow.

Example 2 A solution was prepared by dissolving 17 g. cyclopentanone and 25 g. sodium nitrite in 250 ml. water. This solution was electrolysed in a divided cell at 8-10 C. using a rotating zinc cathode having 50 sq. cm. surface. A current of 1.5 amperes was passed for 11.3 hours while the cathode was rotated at 2000 r.p.rn. A stream of carbon dioxide at the rate of 0.5 liters/min. was continuously introduced into the cathode compartment during the process. At the conclusion of the electrolysing process the resulting cyclopentanone oxime was separated and crystallised as in Example 1. A 48% yield was obtained (9.5 g.) which had a melting point of 56.0-56.5 C. The current elflciency was 62.4%.

Example 3 A solution was prepared comprising 12 g. acetone and 25 g. sodium nitrite in 250 ml. of water and this solution was electrolysed in a divided cell using a zinc cathode of 50 sq. cm. rotated at a speed of 2000 r.p.m. A current of 1.5 amperes was passed for 10 hours. The catholyte was maintained at a temperature of about 7-9 C. Carbon dioxide was introduced at a rate of 0.5 liters/ min. After separating and crystallising the oxime product as in Example 1 the yield or acetoxime was found to be 39.7% and the current etficiency was 56.7%.

Example 4 A solution of 14.5 g. methyl ethyl ketone and 25 g. sodium nitrite in 250 ml. of water was electrolysed as in the preceding examples using a zinc cathode having sq. cm. of surface. A current of 2.0 amperes was passed for 10 hours. The rate of introducing carbon dioxide was 0.6 liters/hr. and 6.5 g. (37.4%) of oxime was obtained, isolation being carried out as in the preceding examples. The current efficiency was 40%.

What is claimed is:

1. A process comprising electrolytically reducing in the cathode compartment of an electrolytic cell with a zinc cathode a solution of a compound from the class consisting of an inorganic nitrite salt and an inorganic nitrate salt in the presence of a compound selected from the class consisting of aliphatic and cycloaliphatic ketones at a pH of from about 5 to 11 to form a ketoxime and maintaining said pH during said reduction process.

2. A process of preparing cyclohexanone oxime comprising electrolytically reducing in the cathode compartment of an electrolytic cell with a zinc cathode a solution of cyclohexanone and sodium nitrite at a pH between about 5 and 11 and a temperature of about 10 to 12 C., and maintaining said pH during said reduction process.

References Cited in the file of this patent UNITED STATES PATENTS Tafel May 3, 1903 OTHER REFERENCES Electrochem. Soc., vol. 28 

1. A PROCESS COMPRISING ELECTROLYTICALLY REDUCING IN THE CATHODE COMPARTMENT OF AN ELECTROLYTIC CELL WITH A ZINC CATHODE A SOLUTION OF A COMPOUND FROM THE CLASS CONSISTING OF AN INORGANIC NITRITE SALT AND AN INORGANIC NITRATE SALT IN THE PRESENCE OF A COMPOUND SELECTED FROM THE CLASS CONSISTING OF ALIPHATIC AND CYCLOALIPHATIC KETONES AT A PH OF FROM ABOUT 5 TO 11 TO FORM A KETOXIME AND MAINTAINING SAID PH DURING SAID REDUCTION PROCESS. 